a) Field of the Invention
The present invention is directed to a process for producing a high temperature stable fiber composite ceramic by gas phase infiltration (CVI=Chemical Vapor Infiltration) with a silicon carbide precursor in a carrier gas suitable for this purpose, preferably on carbon fiber preforms or silicon carbide fiber preforms.
b) Description of the Related Art
Carbon fiber reinforced carbide ceramics or silicon fiber reinforced silicon carbide ceramics have not only proven to be resistant to high temperatures, but are also distinguished by low specific gravity and are therefore suitable as a material for producing thermally and mechanically loaded structural component parts, for example, for recoverable or reentry spacecraft. An example of such structural component parts is a control flap such as that provided for the X-38 experimental space glider or so-called crew return vehicles of the ISS international space station. Other areas of application are the leading edge of blades, nose cones, control rudders and heat protection paneling for spacecraft and hypersonic aircraft.
The chemical vapor infiltration method, or CVI method, for short, has been known for more than twenty years (E. Fitzer and D. Hegen, Angew. Chem. Int. Ed. Engl., 18 (1979), 295-304). It had already been determined at that time that the total deposition rate in the pores could be controlled only by the speed of the chemical reaction rather than by the transport speed in order to achieve good impregnation (see 299, op. cit., left column, first paragraph). It was concluded from this that the deposition must be carried out at low temperatures and with low concentrations. By “low temperatures” is meant temperatures below 1000° C.
Further studies in this field determined that the process temperature to be maintained needed to be adjusted to up to 900<T<1100° C. and the total pressure to 0.1<p<0.6 atm (R. Naslain et al., “The Carbon-Fiber Carbon and Silicon Carbide Binary Matrix Composites. A New Class of Materials for High Temperatures Applications”, Proc. ICCM 3, 3rd Intern. Conf. on Composite Materials, Paris, 1981, 1084-1097). However, it follows from the description of the experimental findings that the conditions actually applied must, in part, be appreciably below the upper limits mentioned above, namely, 900<T<1000° C. and 0.05<p<0.5 atm (R. Naslain et al., “Synthesis and Properties of New Composite Materials for High Temperature Applications Based on Carbon Fibers and C-SiC or C-TiC Hybrid Matrices, Revue de Chimie Minerale”, Volume 18, 1981, 544-564).
The choice of low process parameter values was based on the fact that the life span of the molecular species leading to silicon carbide (SiC) formation is increased and deposition into the depth of the pores is made possible in this way. However, such process parameters cause a very low deposition rate and, therefore, a long process period of several hundred hours for generating a matrix. Nevertheless, their necessity was emphasized again also in R. Naslain, “Fibrous Ceramic-Ceramic Composite Materials for Transport Applications”, Proc. MRS Materials Research Society Meeting, Strasbourg, 1985, 99-115.
The methods according to the prior art are based on process conditions allowing the greatest possible free path length of the gaseous molecular species which takes part in the reaction leading to the deposition of silicon carbide (SiC). This is meant to ensure the penetration of the silicon carbide-forming species into the deeper zones of a fiber preform and accordingly a uniform entry of the SiC matrix over the wall thickness. However, the life of the molecular species essentially coming under consideration is very short in the temperature range to be used for the formation of SiC. This has to do with metastable fragments of starting materials such as SiCl2, SiCl3 or CH3 from which the SiC matrix is formed in a surface reaction on a substrate and in the cavities of a fiber preform. The yield with respect to the adjusted material quantity is also correspondingly small.
This short life means an opposite effect for the steps mentioned above for promoting deep infiltration. The rate of formation of the SiC matrix is determined on the one hand by the transport of the reactive gas species which is limited by low temperature and low partial pressure of the starting material and by the impaired removal of hydrogen chloride (HCl) which is absorbed as a byproduct at the substrate surface and has an inhibiting effect on the SiC-forming surface reaction (F. Langlais, C. Prebende, Proc. 11th Intern. Conf. on Chemical Vapour Deposition, Seattle 1990, eds. K. E. Spear and G. W. Cullen, Electrochem. Soc., 686-959).